Flight control system with low-frequency instrument landing system localizer anomaly detection and method of use

ABSTRACT

A flight control module for detecting anomalies in ILS localizer signals during landing of an aircraft is provided. The flight control module includes a communication interface and a processor coupled thereto. The communication interface is configured to receive inertial data, GPS data, and an ILS localizer deviation for the aircraft. The processor is configured to compute an inertial localizer deviation based on the inertial data and a GPS localizer deviation based on the GPS data. The processor is configured to compare the ILS localizer deviation to an average of the inertial localizer deviation and the GPS localizer deviation to detect a low-frequency anomaly in the ILS localizer deviation. The processor is configured to initiate a transition from controlling the aircraft based on the ILS localizer deviation to controlling the aircraft based on the inertial localizer deviation when the low-frequency anomaly is detected.

BACKGROUND

The field of the disclosure relates generally to flight control systemsand, more specifically, to a flight control module utilizing a syntheticinertial localizer deviation for detecting low-frequency instrumentlanding system (ILS) localizer anomalies.

Many known aircraft feature an automated landing system that controlsthe aircraft during landing. Automated landing systems have becomeincreasingly more common and are frequently relied on for bothinstrument landings under instrument flight rules (IFR) and landingsperformed under visual flight rules (VFR). Known automated landingsystems utilize various receivers, such as multi-mode receivers (MMRs),for example, to receive guidance signals transmitted from the ground.Such guidance signals may include, for example, ILS signals, globalpositioning service (GPS) landing system (GLS) signals, and/or microwavelanding system (MLS) signals. The guidance signals inform the aircraftof its position relative to a desired vertical and lateral path to therunway and through roll-out after touchdown. The desired vertical pathis referred to as the glideslope and the lateral path is referred to asthe localizer. The glideslope is typically defined as a 3° descent witha desired intercept with the ground at 1000 feet beyond the runwaythreshold. The localizer guides the aircraft to the runway centerline.

The guidance signals transmitted from the ground are received by anon-board antenna and routed to redundant MMRs. Each MMR computes alocalizer deviation and a glideslope deviation that are routed to aflight control module that includes the automated landing system. Thelocalizer deviation is an indication of the aircraft's position relativeto the desired path to the runway centerline. For example, the localizerdeviation may indicate the aircraft is approximately 2° left of therunway centerline. The glideslope deviation is an indication of theaircraft's position relative to the desired glideslope to the runway.For example, the glideslope deviation may indicate the aircraft is 1°below the desired glideslope. The flight control module uses thelocalizer deviation and the glideslope deviation to command theautomated landing system and to command control surfaces of theaircraft.

Under IFR conditions, the runway is typically kept clear of potentialdisruptions in the localizer signals transmitted from the ground. Thisis sometimes referred to as protecting the critical area. For example,taxiing aircraft and ground equipment are steered clear of the runwaywhile a landing aircraft is on approach. Similarly, airborne aircraftare prohibited from flying across the airfield. In these situations, thethroughput of the runway and surrounding airfield is reduced. Under VFRconditions, restrictions on operation of the runway and surroundingairfield are more relaxed, i.e., the critical area is not protected,allowing the runway and surrounding airfield to operate with greaterthroughput. More frequent use of automated landing systems under VFRconditions increases the likelihood of disruptions in the localizersignals transmitted from the ground. Such disruptions are referred to asanomalies in the localizer signals. Anomalies may result in drifting ofthe localizer input to automated landing systems, which may furtherresult in misguiding the aircraft during landing and ultimatelytransitioning from automated landing to a manual landing.

BRIEF DESCRIPTION

According to one aspect of the present disclosure, a flight controlmodule for detecting anomalies in instrument landing system (ILS)localizer signals during landing of an aircraft is provided. The flightcontrol module includes a communication interface and a processorcoupled thereto. The communication interface is configured to receiveinertial data, global position system (GPS) data, and an ILS localizerdeviation for the aircraft. The processor is configured to compute aninertial localizer deviation based on the inertial data and a GPSlocalizer deviation based on the GPS data. The processor is configuredto compare the ILS localizer deviation to an average of the inertiallocalizer deviation and the GPS localizer deviation so as to detect alow-frequency anomaly in the ILS localizer deviation. The processor isconfigured to initiate a transition from controlling the aircraft basedon the ILS localizer deviation to controlling the aircraft based on theinertial localizer deviation when the low-frequency anomaly is detected.

According to another aspect of the present disclosure, a flight controlsystem for landing an aircraft is provided. The flight control systemincludes a communication bus, a GPS system, a multi-mode receiver (MMR),and a flight control module. The communication bus is coupled to the GPSsystem, the MMR, and the flight control module. The GPS system isconfigured to transmit a GPS position of the aircraft onto thecommunication bus. The MMR is configured to transmit an ILS localizerdeviation onto the communication bus. The ILS localizer deviation isgenerated according to received ILS localizer signals. The flightcontrol module is configured to receive inertial data for the aircraft,the GPS position, and the ILS localizer deviation on the communicationbus. The flight control module is further configured to compute a GPSlocalizer deviation and an inertial localizer deviation. The flightcontrol module is further configured to detect a low-frequency anomalyin the received ILS localizer signals based on variances in the ILSlocalizer deviation relative to an average localizer deviation computedfor the GPS localizer deviation and the inertial localizer deviation.

According to yet another aspect of the present disclosure, a method ofdetecting a low-frequency anomaly in an instrument landing system (ILS)localizer signal received by an aircraft during landing is provided. Themethod includes receiving an ILS localizer signal. The method includescomputing an ILS localizer deviation based on the ILS localizer signal.The method includes controlling the aircraft according to the ILSlocalizer deviation. The method includes computing a GPS localizerdeviation. The method includes computing an inertial localizerdeviation. The method includes computing an average localizer deviationof the GPS localizer deviation and the inertial localizer deviation. Themethod includes detecting the low-frequency anomaly when a differencebetween the average localizer deviation and the ILS localizer deviationexceeds a threshold. The method includes controlling the aircraftaccording to the inertial localizer deviation after detecting thelow-frequency anomaly.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments or may be combined in yetother embodiments further details of which can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective diagram of an exemplary instrument landingsystem (ILS) for use in landing an aircraft;

FIG. 2 is a side perspective diagram of the exemplary ILS shown in FIG.1;

FIG. 3 is a top perspective diagram of the aircraft shown in FIGS. 1 and2 during landing;

FIG. 4 is block diagram of an exemplary flight control system for theaircraft shown in FIGS. 1-3;

FIG. 5 is a functional block diagram of an exemplary flight controlmodule for use in the flight control system shown in FIG. 4;

FIG. 6 is a functional block diagram of another exemplary flight controlmodule for use in the flight control system shown in FIG. 4; and

FIG. 7 is a flow diagram of an exemplary method of detecting alow-frequency anomaly in an ILS localizer signal received by an aircraftduring landing.

DETAILED DESCRIPTION

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralelements or steps unless such exclusion is explicitly recited.Furthermore, references to “one embodiment” of the present invention orthe “exemplary embodiment” are not intended to be interpreted asexcluding the existence of additional embodiments that also incorporatethe recited features.

FIG. 1 is a perspective diagram, from a top-view, of an exemplaryinstrument landing system (ILS) 100 for use in landing an aircraft 102.FIG. 2 is another perspective diagram, from a side view, of ILS 100.Aircraft 102 is illustrated during approach for landing on a runway 104.Runway 104 is characterized by a runway centerline 106 that extendstowards and beyond aircraft 102 for illustrative purposes.

Referring to FIG. 1, in the exemplary embodiment, ILS 100 includes alocalizer transmitter 108 and a glideslope transmitter 110. Localizertransmitter 108 transmits a first localizer beam 112 and a secondlocalizer beam 114 towards aircraft 102. First localizer beam 112 andsecond localizer beam 114 cooperate to define an ILS localizer signalthat is received by aircraft 102 and processed to generate localizerdeviations indicative of the lateral position of aircraft 102 relativeto runway centerline 106. The localizer signals are typically used tocontrol aircraft 102 through landing and rollout, i.e., decelerationalong runway prior to taxi.

Under certain circumstances, another aircraft 115 or ground equipment(not shown) may travel through the path of first localizer beam 112 orsecond localizer beam 114 while aircraft 102 is on approach. Suchactivity may disrupt first localizer beam 112 or second localizer beam114, introducing multi-path effects in the ILS localizer signals thatare received by aircraft 102. The multi-path effects typically manifestas low-frequency oscillations or high-frequency oscillations in the ILSlocalizer deviations. High-frequency oscillations typically result froman airborne aircraft or other fast moving object moving quickly throughfirst localizer beam 112 or second localizer beam 114. Such oscillationstypically have a period of 1 second or fewer, i.e., one Hertz orgreater. Some known systems for aircraft 102 are configured to detectthese high-frequency anomalies in the ILS localizer signals.Low-frequency oscillations typically result from slow moving vehicles onthe ground, such as, for example, taxiing aircraft and ground equipment.Such oscillations typically have a period greater than 1 second, i.e.,below one Hertz.

Referring to FIG. 2, aircraft 102 approaches runway 104 along apredefined glideslope 116 that intercepts runway 104 at a predefineddistance beyond a runway threshold for runway 104. The predefineddistance is typically, for example, at least 1000 feet from the runwaythreshold, which is typically the site of glideslope transmitter 110.Glideslope 116 is defined by a glideslope angle 118 measured betweenglideslope 116 and runway 104. A typical glideslope, for example, isdefined by glideslope angle 118 being equal to approximately 3°.Glideslope transmitter 110 transmits a first glideslope beam 120 and asecond glideslope beam 122 towards aircraft 102. First glideslope beam120 and second glideslope beam 122 define an ILS glideslope signal thatis received by aircraft 102 and that is processed to generate glideslopedeviations indicative of the vertical position of aircraft 102 relativeto glideslope 116. The glideslope signals are typically used to controlaircraft 102 until it reaches a flare altitude, i.e., an altitude whenthe nose of aircraft 102 pitches up prior to landing, which typicallyoccurs between 50 and 75 feet. When the flare altitude is reached,aircraft 102 typically switches to a radio altimeter to guide aircraft102 to runway 104 for landing. In alternative embodiments, flarealtitude may be greater than 75 feet or, in some embodiments, below 50feet.

FIG. 3 is a perspective diagram of aircraft 102 during landing. FIG. 3illustrates a top-view of aircraft 102 landing on runway 104. Aircraft102 includes a guidance control point (GCP) 302 to which all flightcontrol commands for aircraft 102 are referenced. GCP 302 is typicallylocated at the nose of aircraft 102. Aircraft 102 also includes aninertial reference unit (IRU) 304 that includes various sensors fordetecting linear and angular accelerations of aircraft 102, which aretranslatable to accelerations, velocities, and attitude of aircraft 102along three axis, i.e., pitch, roll, and yaw. IRU 304 is typicallylocated at or near the center of aircraft 102, which is illustrated inFIG. 3 as the intersection of the wings 305 and fuselage 307 of aircraft102. Accordingly, GCP 302 and IRU 304 are typically separated by adistance 306 extending along a portion of the length of fuselage 307 ofaircraft 102.

Runway 104 includes runway centerline 106 characterized by a runwayheading 308 relative to magnetic North (N). Runway heading 308 isgenerally known by aircraft 102 and its navigations systems, and issometimes referred to as a magnetic runway heading. During landing,aircraft 102 travels at a ground speed 310 along a track angle, orsimply track 312, relative to North. Ground speed 310 and track 312 aremeasurable by IRU 304. Moreover, aircraft 102 travels with an aircraftheading 314 relative to North, which is generally defined as thedirection the nose of aircraft 102 is pointing. Aircraft heading 314 isalso measurable by IRU 304. Notably, under certain circumstances, suchas cross-winds, for example, track 312 and aircraft heading 314 may bedifferent.

FIG. 3 illustrates aircraft 102 at an orientation with a localizerdeviation 316 measured from runway centerline 106 to IRU 304. Giveninertial accelerations measured by IRU 304 and runway heading 308,aircraft 102 may compute an inertial cross-runway velocity 318 that,over time, increases or decreases localizer deviation 316 and that canbe translated to GCP 302.

FIG. 4 is block diagram of an exemplary flight control system 400 foraircraft 102, shown in FIGS. 1-3. Flight control system 400 includes aflight control module 402 that controls aircraft 102 by transmittingcommands to an actuator control module 404. Flight control module 402communicates with actuator control module 404 over a communication bus406. Actuator control module 404 controls one or more actuators 408 thatare attached to various flight control surfaces of aircraft 102.Actuator control module 404 communicates with actuators 408 over acommunication bus 410.

Aircraft 102 includes various sensors 412 that measure flight parametersand generate data that is transmitted onto a communication bus 414.Flight control module 402 is communicably coupled to communication bus414 through communication interface 415 and gains access to the data.

Sensors 412 include various accelerometers and gyroscopes located at IRU304 that provide cross-runway acceleration 416, ground speed 310, trackangle 312, and aircraft heading 314. Communication bus 414 is configuredto be coupled to IRU 304, which provides the inertial data.Communication bus 414 is further coupled to various other data sources,such as a navigation system (not shown) that provides runway heading 308and a radar altimeter 418 that provides altitude above ground level foraircraft 102. In alternative embodiments, other measurements of altitudemay be utilized and may be available on aircraft 102, such as, forexample, barometric altitude or GPS ellipsoid altitude; however,altitude above ground level is most relevant to landing aircraft 102.Communication bus 414 is further coupled to a left MMR 420 and a rightMMR 422. Left MMR 420 provides a left MMR localizer deviation 424. RightMMR 422 provides a right MMR localizer deviation 426. Communication bus414 is further coupled to a GPS 430 that provides a GPS position 432 foraircraft 102.

Communication interface 415 receives first and second localizerdeviation signals indicative of respective localizer deviations, such asleft MMR localizer deviation 424 and right MMR deviation 426, computedbased on the localizer transmission received by aircraft 102.

Communication interface 415 is further configured to receive GPSposition 432 over communication bus 414. GPS position 432 is typicallyexpressed as a latitude and longitude.

Flight control module 402 gains access to MMR localizer deviations 424and 426 on communication bus 414 through communication interface 415,which may include an ARINC-429 interface circuit card configured tocommunicate within flight control module 402 using peripheral componentinterconnect (PCI), PCI Express, PC/104, Ethernet, compact PCI, or othersuitable protocol.

Flight control module 402 includes an automated landing system 428.Flight control module 402 receives and processes data from communicationbus 414 to detect low-frequency ILS anomalies in ILS localizer signals.Flight control module 402 includes a complementary filter 434 and a lagfilter 436 for filtering MMR localizer deviations 424 and 426. Flightcontrol module 402 includes an enabling processor 438 that enables ordisables a low-frequency ILS localizer anomaly detection processor 440based on inertial data, MMR localizer deviations 424 and 426, and GPSposition 430.

Enabling processor 438 operates, for example, when aircraft 102 isbetween altitudes of 600 feet and 200 feet. In the event a low-frequencyILS anomaly exists as aircraft 102 descends through 200 feet, enablingprocessor 438 prevents low-frequency ILS localizer anomaly detectionprocessor 440 from engaging and synchronizing to the anomaly. Enablingprocessor 438 generates a disable signal 442 that controls whetherlow-frequency ILS localizer anomaly detection processor 440 will engagewhen aircraft 102 descends below, for example, 200 feet.

Low-frequency ILS localizer anomaly detection processor 440 detectsanomalies based on inertial data, MMR localizer deviations 424 and 426,and GPS position 430. Low-frequency ILS localizer anomaly detectionprocessor 440 generates an anomaly detected signal 444 that is used byflight control module 402 and automated landing system 428 to controlaircraft 102. For example, when an anomaly is detected in MMR localizerdeviations 424 or 426, automated landing system 428 transitions fromcontrol based on MMR localizer deviations 424 and 426 to control basedon an inertial localizer deviation.

FIG. 5 is a functional block diagram of flight control module 402 and,more specifically, a synthetic inertial localizer deviation (SILD)processor 500. SILD processor 500 includes a complementary filter 502, arunway heading correction block 504, an inertial cross-runway velocityblock 506, and a translation-to-GCP block 508.

Complementary filter 502 blends high-frequency content of inertial datafrom IRU 304 with low-frequency content of an ILS localizer deviation501, such as MMR localizer deviations 424 and 426, to produce a smooth,complementary-filtered localizer deviation 503. Complementary-filteredlocalizer deviation 503 is fed back and subtracted 510 from ILSlocalizer deviation 501 to produce a localizer deviation error value.The localizer deviation error value is gained by K₃ and integrated 512.The result of integration 512 is added 514 to cross-runway acceleration416, and then added 516 to the localizer deviation error value gained518 by K₂. The result of summing 516 is integrated 520 and added 522 tothe localizer deviation error value gained 524 by K₁. The result ofsummation 522 is a localizer deviation rate 526 that is integrated 528to generate complementary-filtered localizer deviation 503.Complementary filter 502 is further characterized by, but not limited tothe following equation, where D_(CF) is complementary-filtered localizerdeviation 503 as a function of time, t, and expressed in radians,D_(sel) is ILS localizer deviation 501 as a function of time, t, andexpressed in radians, and A_(CR) is cross-runway acceleration 416 as afunction of time, t.

$\begin{matrix}{D_{CF} = {{\left\lbrack \frac{{K_{1}S^{2}} + {K_{2}S} + K_{3}}{S^{3} + {K_{1}S^{2}} + {K_{2}S} + K_{3}} \right\rbrack \times D_{sel}} + {\left\lbrack \frac{S}{S^{3} + {K_{1}S^{2}} + {K_{2}S} + K_{3}} \right\rbrack \times A_{CR}}}} & {{EQ}.\mspace{14mu} 1}\end{matrix}$

When aircraft 102 descends, as measured by radar altimeter 418, below analtitude threshold 530, complementary-filtered localizer deviation 503is latched 532 as an initial condition for integration 534 of inertialcross-runway velocity 318. Aircraft altitude from radar altimeter 418 iscompared 536 to altitude threshold 530, e.g., 200 feet, to trigger latch532.

Inertial cross-runway velocity block 506 computes inertial cross-runwayvelocity 318 as a function of an adjusted runway heading 538, groundspeed 310, and track 312. Inertial cross-runway velocity 318 is aprojection of ground speed 310 along track 312 onto a cross-runwayvector; computed as ground speed 310 multiplied 540 by the sine 542 ofthe difference 544 between track 312 and adjusted runway heading 538.Inertial cross-runway velocity block 506 is further characterized by,but not limited to, the following equation, where V_(CR) is cross-runwayvelocity 318 as a function of time, t, S_(GND) is ground speed 310 as afunction of time, t, T is track 312 as a function of time, t, andH_(run,adj) is adjusted runway heading 538 as a function of time, t, andexpressed in radians.V _(CR)(t)=S _(GND)(t)×sin(T(t)−H _(run,adj)(t))  EQ. 2

Runway heading correction block 504 computes adjusted runway heading 538to correct for errors in magnetic runway heading 308 available onaircraft 102. Adjusted runway heading 538 represents actual runwayazimuth relative to North for use in inertial cross-runway velocityblock 506 and translation-to-GCP block 508. Adjusted runway heading 538is computed as a function of magnetic runway heading 308, ground speed310, track 312, and localizer deviation rate 526 from complementaryfilter 502. Localizer deviation rate 526 is derived from EQ. 1, above,and is a component of complementary-filtered localizer deviation 503.Localizer deviation rate 526 is a derivative of complementary-filteredlocalizer deviation 503, i.e., S×D_(CF), and is represented by thefollowing equation, where D_(rate) is localizer deviation rate 526 as afunction of time, t, and expressed in radians per second.

$\begin{matrix}{D_{rate} = {{\left\lbrack \frac{{K_{1}S^{2}} + {K_{2}S^{2}} + {K_{3}S}}{S^{3} + {K_{1}S^{2}} + {K_{2}S} + K_{3}} \right\rbrack \times D_{sel}} + {\left\lbrack \frac{S^{2}}{S^{3} + {K_{1}S^{2}} + {K_{2}S} + K_{3}} \right\rbrack \times A_{CR}}}} & {{EQ}.\mspace{14mu} 3}\end{matrix}$

In runway heading correction block 504, localizer deviation rate 526 isdivided 546 by ground speed 310 and subtracted 548, along with magneticrunway heading 308, from track 312. The result of subtraction 548 islag-filtered 550, i.e., low-pass filtered, with a time constant, τ. Theresult of lag-filtering 550 is added 552 to magnetic runway heading 308to generate adjusted runway heading 538. Runway heading correction block504 is further characterized, but not limited to, the followingequation, where H_(run) is magnetic runway heading 308 and is expressedin radians.

$\begin{matrix}{H_{{run},{adj}} = {{\frac{\tau\; S}{{\tau\; S} + 1} \times H_{run}} + {\frac{1}{{\tau\; S} + 1} \times T} - {\frac{1}{{\tau\; S} + 1} \times \frac{D_{rate}}{s_{GND}}}}} & {{EQ}.\mspace{14mu} 4}\end{matrix}$

Integration 534 integrates cross-runway velocity 318 from an initialcondition at altitude threshold 530, which is latched atcomplementary-filtered localizer deviation 503 with respect to GCP 302.Integration 534 produces an inertial localizer deviation 535 withrespect to IRU 304. A compensation 554 is added 556 to inertiallocalizer deviation 535 to correct for the difference between inertiallocalizer deviation 535 at IRU 304 and inertial localizer deviation 535at GCP 302. For example, GCP 302, at the nose of aircraft 102, is at adifferent cross-runway position than IRU 304 during “crabbed” approachesfor cross-wind landings. Compensation 554 accounts for changes inaircraft heading 314 that occur below altitude threshold 530, becauseintegration 534 is initialized, at altitude threshold 530, tocomplimentary-filtered localizer deviation 503, which is computed withrespect to GCP 302.

Translation-to-GCP block 508 computes compensation 554 as a function ofaircraft heading 314, adjusted runway heading 538, and distance 306along the fuselage of aircraft 102 between IRU 304 and GCP 302. Distance306 is multiplied 558 by the sine 560 of a difference 562 betweenaircraft heading 314 and adjusted runway heading 538, yieldingcompensation 554 as a function of time, t. When aircraft 102 descends toaltitude threshold 530, a hold value 564 of compensation 554 is latched532. Hold value 564 represents the portion of compensation 554 alreadyincorporated into inertial localizer deviation 535 via the initialcondition of complementary-filtered localizer deviation 503 latched 532at altitude threshold 530. Hold value 564 is subtracted 566 fromcompensation 554 to capture only the changes in aircraft heading 314that occur below altitude threshold 530. Translation-to-GCP block 508 isfurther characterized, but not limited to, the following equation, whereC is compensation 554 as a function of time, t, L_(IRU-GCP) is distance306 along the fuselage of aircraft 102 between IRU 304 and GCP 302,H_(ac) is aircraft heading 314 as a function of time, t, and t₂₀₀ is thetime at which aircraft 102 descends to altitude threshold 530.C(t)=L _(IRU-GCP)[sin(H _(ac)(t)−H _(run,adj)(t))−sin(H _(ac)(t ₂₀₀)−H_(run,adj)(t ₂₀₀))]  EQ. 5

FIG. 6 is a functional block diagram of flight control module 402 foruse in flight control system 400 shown in FIG. 4. Flight control module402 includes enabling processor 438 and low-frequency ILS localizeranomaly detection processor 440. Low-frequency ILS localizer anomalydetection processor 440 detects anomalies once aircraft has descendedbelow a low approach altitude threshold, for example, 200 feet. Enablingprocessor 438 is operable when aircraft has descended below a highapproach altitude threshold and either enables or disables low-frequencyILS localizer anomaly detection processor 440 based on anomaly detectionbetween the low approach altitude threshold and the high approachaltitude threshold. In one embodiment, for example, the low approachaltitude threshold is 200 feet and the high approach altitude thresholdis 600 feet. In certain embodiments, enabling processor is operablebetween the high approach altitude threshold and just below the lowapproach altitude threshold. For example, where the low approachaltitude threshold is 200 feet and the high approach altitude thresholdis 600 feet, enabling processor 438 is operable from 600 feet to 190feet.

Enabling processor 438 includes a SILD processor 602, such as SILDprocessor 500 shown in FIG. 5, and a GPS localizer deviation processor604. SILD processor 602 computes inertial localizer deviation 535 as afunction of complementary-filtered localizer deviation 503, ground speed310, track 312, aircraft heading 314, and runway heading 308. GPSlocalizer deviation processor 604 computes a GPS localizer deviation 606as a function of runway heading 308, complementary-filtered localizerdeviation 503, and GPS position 432, shown in FIG. 4. GPS position 432is represented by a GPS latitude 608 and a GPS longitude 610. SILDprocessor 602 and GPS localizer deviation processor 604 are initializedwhen aircraft 102 descends to the high approach altitude threshold tothe value of complementary-filtered localizer deviation 503 at the highapproach altitude. For example, SILD processor 602 and GPS localizerdeviation processor 604 are initialized to the value ofcomplementary-filtered localizer deviation 503 when aircraft 102 reaches600 feet altitude above ground level.

Enabling processor 438 includes an average module 612 that computes anaverage localizer deviation 614 as an average of inertial localizerdeviation 535 and GPS localizer deviation 606. Average localizerdeviation 614 is compared to a lag-filtered localizer deviation 616 todetect low-frequency ILS localizer anomalies. Lag-filtered localizerdeviation 616 is a product of passing ILS localizer deviation 501through lag filter 436. Enabling processor 438 compares 618 an absolutevalue, or magnitude 620, of a difference 622 between lag-filteredlocalizer deviation 616 and average localizer deviation 614 to athreshold 624. Generally, if magnitude 620 exceeds threshold 624,enabling processor 438 declares an anomaly. Threshold 624 is generallyin the range of 25 to 100 feet. For example, in one embodiment,threshold 624 is 50 feet. Enabling processor 438 utilizes lag filter 436to smooth ILS localizer deviation 501 rather than complementary filter434, because complementary filter 434 blends inertial data with ILSlocalizer deviation 501 and would slow divergence of the ILS localizerdeviation 501, and further would impede detection of low-frequencyanomalies.

Enabling processor 438 includes a comparator 626 that compares inertiallocalizer deviation 535 and GPS localizer deviation 606. Comparator 626checks that inertial localizer deviation 535 and GPS localizer deviation606 agree within a threshold. If inertial localizer deviation 535 andGPS localizer deviation 606 disagree enough, enabling processor 438disables low-frequency ILS localizer anomaly detection processor 440 viadisable signal 442.

Enabling processor 438 includes various logic to generate disable signal442 and an anomaly detected signal 628. Anomaly detected signal 628 islogically true when inertial localizer deviation 535 and GPS localizerdeviation 606 agree within the threshold, as determined by comparator626, and when comparison 618 of difference 622 to threshold 624 is true.Anomaly detected signal 628 is generated by an and-gate 630. Enablingprocessor 438 includes a time delay 632 to ensure difference 622 betweenlag-filtered localizer deviation 616 and average localizer deviation 614exceeds threshold 624 for at least a minimum duration of time beforedeclaring an anomaly. Enabling processor 438 includes an or-gate 634 tocontrol disabling signal 442. Disabling signal 442 is logically truewhen an anomaly is detected, as indicated by anomaly detected signal628, or when comparator 626 indicates inertial localizer deviation 535and GPS localizer deviation 606 disagree.

Low-frequency ILS localizer anomaly detection processor 440 includes aSILD processor 636, such as SILD processor 500 shown in FIG. 5, and aGPS localizer deviation processor 638. SILD processor 636 computesinertial localizer deviation 535 as a function of complementary-filteredlocalizer deviation 503, ground speed 310, track 312, aircraft heading314, and runway heading 308. GPS localizer deviation processor 604computes GPS localizer deviation 606 as a function of runway heading308, complementary-filtered localizer deviation 503, and GPS position432, shown in FIG. 4. GPS position 432 is represented by a GPS latitude608 and a GPS longitude 610. SILD processor 636 and GPS localizerdeviation processor 638 are initialized when aircraft 102 descends tothe low approach altitude threshold to the value ofcomplementary-filtered localizer deviation 503 at the low approachaltitude. For example, SILD processor 636 and GPS localizer deviationprocessor 638 are initialized to the value of complementary-filteredlocalizer deviation 503 when aircraft 102 reaches 200 feet altitudeabove ground level.

Low-frequency ILS localizer anomaly detection processor 440 includes anaverage module 640 that computes average localizer deviation 614 as anaverage of inertial localizer deviation 535 and GPS localizer deviation606. Average localizer deviation 614 is compared to lag-filteredlocalizer deviation 616 to detect low-frequency ILS localizer anomalies.Low-frequency ILS localizer anomaly detection processor 440 compares 642an absolute value 644 of a difference 646 between lag-filtered localizerdeviation 616 and average localizer deviation 614 to a threshold 648.Generally, if difference 646 exceeds threshold 648, Low-frequency ILSlocalizer anomaly detection processor 440 declares an anomaly. Threshold648 is generally in the range of 1 to 50 feet. For example, in oneembodiment, threshold 648 is 25 feet. Low-frequency ILS localizeranomaly detection processor 440, as in enabling processor 438, utilizeslag filter 436 to smooth ILS localizer deviation 501 rather thancomplementary filter 434.

Low-frequency ILS localizer anomaly detection processor 440 includes acomparator 650 that compares inertial localizer deviation 535 and GPSlocalizer deviation 606. Comparator 650 checks that inertial localizerdeviation 535 and GPS localizer deviation 606 agree within a threshold.If inertial localizer deviation 535 and GPS localizer deviation 606disagree enough, low-frequency ILS localizer anomaly detection processor440 cannot use comparison 642 to detect low-frequency ILS localizeranomalies.

Low-frequency ILS localizer anomaly detection processor 440 includes anand-gate 652 to enforce several necessary conditions for detection oflow-frequency ILS localizer anomalies. The first condition is thatdisable signal 442 is false, generally indicating that enablingprocessor 438 has not disabled low-frequency ILS localizer anomalydetection processor 440. More specifically, disabling signal 442 shouldindicate enabling processor 438 has not detected a low-frequency ILSlocalizer anomaly in its operative range of altitudes, i.e., between ahigh approach altitude threshold and a low approach altitude threshold,e.g., below 600 feet and above 200 feet. The second condition is thatcomparator 650 indicates inertial localizer deviation 535 and GPSlocalizer deviation 606 agree within the appropriate threshold.

A third condition is that aircraft 102 has not completed performance oflanding and rollout. Low-frequency ILS localizer anomaly detectionprocessor 440 includes a time delay 654 that is triggered when aircraft102 touches down. Touch down of aircraft 102 is indicated by a touchdown signal 656 that is received, for example, through communicationinterface 415 and communication bus 414. Touch down signal 656 indicatestouch down of aircraft 102, for example, by a logic-high signal. Timedelay 654 ensures low-frequency ILS localizer anomaly detectionprocessor 440 remains enabled for a predetermined duration after touchdown and during rollout. When the predetermined duration elapses, timedelay 654 negates the third condition for detection of low-frequency ILSlocalizer anomalies.

Given that (1) low-frequency ILS localizer anomaly detection processor440 is not disabled by enabling processor 438, (2) inertial localizerdeviation 535 and GPS localizer deviation 606 agree, and (3) aircraft102 has not completed landing and rollout, low-frequency ILS localizeranomaly detection processor 440 is free to carry out anomaly detection.When comparison 642 detects an anomaly, the detection is latched 658 andanomaly detected signal 444 indicates a low-frequency ILS localizeranomaly has been detected. In response, flight control module 402 maycarry out one or more remedial steps. In certain embodiments, flightcontrol module 402 transitions control of aircraft 102, by automatedlanding system 428, from ILS localizer-based control, i.e., based oncomplementary-filtered localizer deviation 503, to inertiallocalizer-based control, i.e., based on inertial localizer deviation535. In certain embodiments, flight control module 402 may transmit anindicator signal to a pilot interface system (not shown) to notify apilot of aircraft 102 of the detected anomaly. In such embodiments,control of aircraft 102 may ultimately be transitioned from automatedlanding system 428 to the pilot. The pilot interface system may relayanomaly detected signal 444 to the pilot using one or more of visualindicators and aural indicators.

Generally, low-frequency ILS localizer anomalies are temporary,presenting and resolving themselves within the time frame of theapproach by aircraft 102. Low-frequency ILS localizer anomaly detectionprocessor 440 includes the ability to recover from the detectedlow-frequency ILS localizer anomaly through a “healing” procedure.Low-frequency ILS localizer anomaly detection processor 440 includes acomparison 660 of the difference 646 between average localizer deviation614 and lag-filtered ILS localizer 616 to a healing threshold 662. Whena low-frequency ILS localizer anomaly is latched 658, the anomaly can bereset 662 if difference 646 falls below healing threshold 662.Low-frequency ILS localizer anomaly detection processor 440 includes atime delay 664 to ensure difference 646 remains below healing threshold662 for a predetermined duration before negating anomaly detected signal444. Healing threshold 662 is generally in the range of 1 to 50 feet,and is typically less than threshold 648 for initially declaring thelow-frequency ILS localizer anomaly. For example, difference 646 mustrise above threshold 648, e.g., 25 feet, to declare the low-frequencyILS localizer anomaly, and difference 646 must fall below healingthreshold 662, e.g., 15 feet, to negate that low-frequency ILS localizeranomaly. Moreover, difference 646 must fall below healing threshold 662for the predetermined duration defined by time delay 664 before negatingthe low-frequency ILS localizer anomaly.

FIG. 7 is a flow diagram of a method 700 of detecting a low-frequencyanomaly in an ILS localizer signal received by aircraft 102 duringlanding. Method 700 may be embodied in or carried out by flight controlsystem 400 and flight control module 402, shown in FIG. 4. Method 700begins when an ILS localizer signal is received 710 by aircraft 102.Aircraft 102 routes the ILS localizer signal to left MMR 420 and rightMMR 422, which compute 720 ILS localizer deviations, i.e., left MMRlocalizer deviation 424 and right MMR localizer deviation 426, based onthe received ILS localizer signal. Flight control module 402 selectscomplementary-filtered localizer deviation 503 for use in controlling730 aircraft 102 using automated landing system 428 during landing.

Flight control module 402 and, more specifically, low-frequency ILSlocalizer anomaly detection processor 440 and GPS localizer deviationprocessor 638 compute 740 GPS localizer deviation 606 based on GPSposition 432 of aircraft 102 with respect to runway centerline 106.Flight control module 402 and, more specifically, low-frequency ILSlocalizer anomaly detection processor 440 and SILD processor 636 compute750 inertial localizer deviation 535 based on inertial data from IRU304. In alternative embodiments, GPS localizer deviation processor 638and SILD processor 636 are embodied in another device other that flightcontrol module 402, where automated landing system 428 resides. Forexample, in one embodiment, low-frequency ILS localizer anomalydetection processor 440 is embodied in left MMR 420, right MMR 422, orboth. Similarly, enabling processor 438 may be embodied in left MMR 420,right MMR 422, or both, for example.

Low-frequency ILS localizer anomaly detection processor 440 averages 760GPS localizer deviation 606 and inertial localizer deviation 535 forcomparison 642 to lag-filtered localizer deviation 616. A low-frequencyanomaly is detected 770 when difference 646 between lag-filteredlocalizer deviation 616 and average localizer deviation 614 exceedsthreshold 648. When the low-frequency anomaly is detected, control ofaircraft 102 is transitioned from being ILS localizer-based to inertiallocalizer-based. Aircraft 102 is then controlled 780 according toinertial localizer deviation 535 until manual control of aircraft 102 isassumed by a pilot.

In certain embodiments, method 700 includes transmitting an indicator,based on anomaly detected signal 628 and/or anomaly detected signal 444to a pilot interface system, the indicator indicative of detection ofthe low-frequency anomaly. The pilot interface system may, in certainembodiments, relay the indicator to a pilot in the form of a visualindicator, an aural indicator, or both.

In certain embodiments, computing 750 inertial localizer deviation 535includes initializing inertial localizer deviation 535 tocomplementary-filtered localizer deviation 503 when aircraft 102descends below a low approach altitude threshold, e.g., 200 feet.Computing 750, in such embodiments, further includes integratinginertial data, such as, for example, ground speed 310, track angle 312,cross-runway acceleration 416, and aircraft heading 314 on communicationbus 414.

In certain embodiments, method 700 includes initializing SILD processor602 to complementary-filtered localizer deviation 503 when aircraft 102descends below a high approach altitude threshold, e.g., 600 feet. Insuch embodiments, method 700 includes initializing GPS localizerdeviation processor 604 to complementary-filtered localizer deviation503 when aircraft 102 descends below the high approach altitudethreshold. In such embodiments, method 700 further includes detecting,by enabling processor 438, an earlier low-frequency anomaly based onvariances between lag-filtered localizer deviation 616 and averagelocalizer deviation 614, which is averaged 612 based on inertiallocalizer deviation 535 and GPS localizer deviation 606. When theearlier low-frequency anomaly is detected, enabling processor 438disables low-frequency ILS localizer anomaly detection processor 440before aircraft 102 descends beneath the low approach altitudethreshold.

In certain embodiments, method 700 includes computing a differencebetween GPS localizer deviation 606 and inertial localizer deviation535, and disabling detection of the low-frequency anomaly when thedifference exceeds a threshold. Such embodiments carry out thecomparison in comparator 626 and/or comparator 650.

The above described embodiments of flight control systems for use by anaircraft during landing provide low-frequency ILS localizer anomalydetection. More specifically, embodiments described herein provide aflight control module that utilizes a combination of an inertiallocalizer deviation and a GPS localizer deviation to compare with an ILSlocalizer deviation to detect low-frequency anomalies. For example, ifthe ILS localizer deviation differs from an average of the GPS localizerdeviation and the inertial localizer deviation, an anomaly is detected.Embodiments described herein further provide a monitoring function by anenabling processor that detects low-frequency anomalies below a highapproach altitude threshold, e.g., 600 feet down to below a low approachaltitude threshold, e.g., 190 feet. When a low-frequency anomaly isdetected by the enabling processor, detection below the low approachaltitude threshold, e.g., 200 feet, is disabled to preventsynchronization of the computations to the anomaly itself rather than tothe actual distance from runway centerline. Detection of low-frequencyanomalies below the low approach altitude threshold is carried outsimilarly to the monitoring from 600 feet to 190 feet, and generates ananomaly detected signal that can be used to transition control of theaircraft from ILS localizer-based to inertial localizer-based. Further,embodiments described herein provide visual indicators, auralindicators, or both to a pilot of the aircraft to alert them of thelow-frequency anomaly such that the landing can be performed manually.

Exemplary embodiments of methods, systems, and apparatus for flightcontrol systems are not limited to the specific embodiments describedherein, but rather, components of systems and/or steps of the methodsmay be utilized independently and separately from other componentsand/or steps described herein. For example, the methods may also be usedin combination with other non-conventional flight control systems, andare not limited to practice with only the systems and methods asdescribed herein. Rather, the exemplary embodiment can be implementedand utilized in connection with many other applications, equipment, andsystems that may benefit from increased efficiency, reduced operationalcost, and reduced capital expenditure.

An exemplary technical effect of the methods, systems, and apparatusdescribed herein includes at least one of: (a) detecting low-frequencyILS localizer anomalies during landing of an aircraft; (b) transitioningfrom ILS localizer deviation-based control of the aircraft during anautomated landing to a SILD-based control of the aircraft whenlow-frequency ILS localizer anomalies are detected; (c) improvingavailability of automated landing systems through low-frequency ILSlocalizer anomaly detection; (d) reducing false positive detection oflow-frequency ILS localizer anomalies through verification of SILD datawith GPS localizer deviation data; (e) improving localizer guidancebelow 200 feet through monitoring ILS localizer signals forlow-frequency anomalies from 600 to below 200 feet; and (f) providing anindicator of detected low-frequency ILS localizer anomalies via one ormore visual or aural indicators.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor, processing device,or controller, such as a general purpose central processing unit (CPU),a graphics processing unit (GPU), a microcontroller, a reducedinstruction set computer (RISC) processor, an application specificintegrated circuit (ASIC), a programmable logic circuit (PLC), a fieldprogrammable gate array (FPGA), a digital signal processing (DSP)device, and/or any other circuit or processing device capable ofexecuting the functions described herein. The methods described hereinmay be encoded as executable instructions embodied in a computerreadable medium, including, without limitation, a storage device and/ora memory device. Such instructions, when executed by a processingdevice, cause the processing device to perform at least a portion of themethods described herein. The above examples are exemplary only, andthus are not intended to limit in any way the definition and/or meaningof the terms processor, processing device, and controller.

In the embodiments described herein, memory may include, but is notlimited to, a computer-readable medium, such as a random access memory(RAM), and a computer-readable non-volatile medium, such as flashmemory. Alternatively, a floppy disk, a compact disc—read only memory(CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc(DVD) may also be used. Also, in the embodiments described herein,additional input channels may be, but are not limited to, computerperipherals associated with an operator interface such as a mouse and akeyboard. Alternatively, other computer peripherals may also be usedthat may include, for example, but not be limited to, a scanner.Furthermore, in the exemplary embodiment, additional output channels mayinclude, but not be limited to, an operator interface monitor.

This written description uses examples to disclose various embodiments,which include the best mode, to enable any person skilled in the art topractice those embodiments, including making and using any devices orsystems and performing any incorporated methods. The patentable scope isdefined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

What is claimed is:
 1. A flight control module for detecting anomaliesin instrument landing system (ILS) localizer signals during landing ofan aircraft, comprising: a communication interface configured to receiveinertial data for said aircraft, global positioning system (GPS) datafor said aircraft, and an ILS localizer deviation; and a processorcoupled to said communication interface and configured to: compute aninertial localizer deviation based on the inertial data, compute a GPSlocalizer deviation based on the GPS data, compare the ILS localizerdeviation to an average of the inertial localizer deviation and the GPSlocalizer deviation to detect a low-frequency anomaly in the ILSlocalizer deviation, and initiate a transition from controlling saidaircraft based on the ILS localizer deviation to controlling saidaircraft based on the inertial localizer deviation when thelow-frequency anomaly is detected.
 2. The flight control module of claim1, wherein said communication interface is further coupled to a radaraltimeter configured to measure altitude of said aircraft above groundlevel, and wherein said processor is further configured to initializecomputation of the inertial localizer deviation and the GPS localizerdeviation to the ILS localizer deviation when said aircraft descendsbeneath a low approach altitude threshold.
 3. The flight control moduleof claim 2, wherein said processor is further configured to: initializea second computation of the inertial localizer deviation and the GPSlocalizer deviation when said aircraft descends beneath a high approachaltitude threshold; compare the second computations to the ILS localizerdeviation to detect an earlier low-frequency anomaly when said aircraftis located between the high approach altitude threshold and the lowapproach altitude threshold; and disable detection of the low-frequencyanomaly below the low approach altitude threshold when the earlierlow-frequency anomaly is detected between the high approach altitudethreshold and the low approach altitude threshold.
 4. The flight controlmodule of claim 3, wherein the low approach altitude threshold is 200feet and the high approach altitude threshold is 600 feet.
 5. The flightcontrol module of claim 1, wherein said processor is further configuredto: compare the inertial localizer deviation to the GPS localizerdeviation; and disable detection of the low-frequency anomaly when adifference between the inertial localizer deviation and the GPSlocalizer deviation exceeds a threshold.
 6. The flight control module ofclaim 1, wherein the processor is further configured to: compute theaverage of the inertial localizer deviation and the GPS localizerdeviation; compute an absolute value difference between the average andthe ILS localizer deviation; and latch an anomaly indicator when theabsolute value difference exceeds a trip threshold.
 7. The flightcontrol module of claim 6, wherein the processor is further configuredto reset said anomaly indicator after the absolute value differencefalls below a healing threshold for a healing duration.
 8. The flightcontrol module of claim 1, wherein the processor is further configuredto initiate an indication to a pilot of said aircraft that thelow-frequency anomaly has been detected.
 9. A flight control system forlanding an aircraft, said flight control system comprising: acommunication bus; a global positioning system (GPS) coupled to saidcommunication bus and configured to transmit a GPS position of saidaircraft onto said communication bus; a multi-mode receiver (MMR)coupled to said communication bus and configured to transmit aninstrument landing system (ILS) localizer deviation onto saidcommunication bus, the ILS localizer deviation generated according toreceived ILS localizer signals; a flight control module comprising aprocessor coupled to said communication bus and configured to: receiveinertial data for said aircraft, the GPS position, and the ILS localizerdeviation on said communication bus, compute a GPS localizer deviationand an inertial localizer deviation, and detect a low-frequency anomalyin the received ILS localizer signals based on variances in the ILSlocalizer deviation relative to an average localizer deviation computedfor the GPS localizer deviation and the inertial localizer deviation;and an automated landing system configured to transition fromcontrolling said aircraft based on the ILS localizer deviation tocontrolling said aircraft based on the inertial localizer deviation whenthe low-frequency anomaly is detected.
 10. The flight control system ofclaim 9, wherein said flight control module is further configured totransmit an indicator to a pilot interface system, the indicatorindicative of detection of the low-frequency anomaly.
 11. The flightcontrol system of claim 10, wherein said pilot interface system isconfigured to present a visual indicator to a pilot in response to theindicator received from said flight control module.
 12. The flightcontrol system of claim 10, wherein said pilot interface system isconfigured to present an aural indicator to a pilot in response to theindicator received from said flight control module.
 13. The flightcontrol system of claim 9, wherein said flight control module is furtherconfigured to: enable detection of the low-frequency anomaly when saidaircraft descends below a low approach altitude threshold; and disabledetection of the low-frequency anomaly below the low approach altitudethreshold when the low-frequency anomaly is otherwise detected above thelow approach altitude threshold.
 14. The flight control system of claim9, wherein said flight control module is further configured to disabledetection of the low-frequency anomaly when a difference between the GPSlocalizer deviation and the inertial localizer deviation exceeds athreshold.
 15. The flight control system of claim 9, wherein said flightcontrol module is further configured to disable detection of thelow-frequency anomaly after a rollout duration beginning when saidaircraft touches down.
 16. The flight control system of claim 9, whereinsaid flight control module is further configured to: initiatecomputation of the GPS localizer deviation and the inertial localizerdeviation based on a complementary-filtered ILS localizer deviation; andcompare the average localizer deviation to a lag-filtered ILS localizerdeviation to detect the low-frequency anomaly.
 17. The flight controlsystem of claim 9, wherein said flight control module is furtherconfigured to receive inertial data including ground speed, track angle,cross-runway acceleration, and aircraft heading on said communicationbus.
 18. A method of detecting a low-frequency anomaly in an instrumentlanding system (ILS) localizer signal received by an aircraft duringlanding, said method comprising: receiving, at a multi-mode receiver(MMR), an instrument landing system (ILS) localizer signal; computing,by the MMR, a ILS localizer deviation based on the ILS localizer signal;controlling, by an automated landing system, the aircraft according tothe ILS localizer deviation; computing, by a flight control module, aglobal position system (GPS) localizer deviation; computing, by theflight control module, an inertial localizer deviation; computing, bythe flight control module, an average localizer deviation of the GPSlocalizer deviation and the inertial localizer deviation; detecting, bythe flight control module, the low-frequency anomaly when a differencebetween the average localizer deviation and the ILS localizer deviationexceeds a threshold; and controlling, by the automated landing system,the aircraft according to the inertial localizer deviation afterdetecting the low-frequency anomaly.
 19. The method of claim 18, whereincomputing the inertial localizer deviation comprises: initializing theinertial localizer deviation to the ILS localizer deviation when theaircraft descends below a low approach altitude threshold; andintegrating inertial data for the aircraft to continuously update theinertial localizer deviation below the low approach altitude thresholdand through rollout.
 20. The method of claim 19 further comprising:initializing, by the flight control module, a second inertial localizerdeviation to the ILS localizer deviation when the aircraft descendsbelow a high approach altitude threshold; initializing, by the flightcontrol module, a second GPS localizer deviation to the ILS localizerdeviation when the aircraft descends below the high approach altitudethreshold; detecting, by the flight control module, an earlierlow-frequency anomaly based on variances between the ILS localizerdeviation and a second average localizer deviation of the secondinertial localizer deviation and the second GPS localizer deviation; anddisabling, by the flight control module, detection of the low-frequencyanomaly before the aircraft descends beneath the low approach altitudethreshold.
 21. The method of claim 18 further comprisingcomplementary-filtering the ILS localizer deviation for initializing theinertial localizer deviation and for controlling the aircraft.
 22. Themethod of claim 18 further comprising: computing, by the flight controlmodule, a difference between the GPS localizer deviation and theinertial localizer deviation; and disabling, by the flight controlmodule, detection of the low-frequency anomaly when the differenceexceeds a threshold.
 23. The method of claim 18 further comprising:lag-filtering, by the flight control module, the ILS localizerdeviation; and computing, by the flight control module, a differencebetween the average localizer deviation and the lag-filtered ILSlocalizer deviation for detection of the low-frequency anomaly.
 24. Themethod of claim 18 further comprising transmitting, by the flightcontrol module, an indicator to a pilot interface system, the indicatorindicative of detection of the low-frequency anomaly.